Mature blood cells are produced by the clonal proliferation and concomitant differentiation of immature precursor cells (Metcalf, D. (1984) The Hemopoietic Colony Stimulating Factors, Elsevier, Amsterdam.). For normal haemopoietic cells, the two processes of proliferation and differentiation are tightly coupled so that the short-lived mature cells are continually replenished. At least four biochemically distinct growth factors, the colony-stimulating factors (CSFs), have been shown in vitro to stimulate the proliferation and differentiation of precursor cells of granulocytes and macrophages: G-CSF, M-CSF, GM-CSF and Multi-CSF (IL-3) (see Metcalf, D. (1987), Proc.R.Soc.B. 230, 389-423, for review).
In contrast to normal cells, leukaemic myeloid cells are characterized by an uncoupling of proliferation and differentiation so that immature progenitor cells accumulate, retain their proliferative capacity and fail to differentiate. There has been considerable controversy concerning the nature of biological factors that are able to induce the differentiation of myeloid leukaemic cells in vitro and whether certain factors are capable of inducing differentiation in the absence of proliferation. Indeed, it has been proposed that the events of proliferation and differentiation are necessarily mediated by different factors (Sachs, L. (1982), J.Cell.Physiol.Suppl., 1, 151-164). On the one hand, two groups have described and purified an activity (MGI-2 or D-factor) capable of inducing the differentiation of the murine myeloid leukaemic cell line M1, that does not stimulate the proliferation of normal progenitor cells (Lipton, J. H. and Sachs, L. (1981), Biochem.Biophys, Acta. 673, 552-569; Tomida, M., Yamamoto-Yamaguchi, Y., and Hozumi, M. (1984) J.Biol.Chem. 2, 10978-10982). On the other hand, the present inventors have previously shown that one of the CSFs, G-CSF, is a strong differentiation-inducing stimulus for the murine myeloid leukaemic cell line, WEHI-3B D.sup.+, as well as being a proliferative and differentiative stimulus for normal cells (Nicola, N. A., Metcalf, D., Matsumoto, M. and Johnson, G. R. (1983), J.Biol.Chem. 258, 9017-9023). Moreover, Tomida, et.al. (Tomida, M., Yamamoto-Yamaguchi, Y., Hozumi, M., Okabe, T. and Takaka, F. (1986), FEBS Lett., 207, 271-275) have shown that recombinant G-CSF is also able to induce the macrophage differentiation of M1 cells. The relationship between these factors has been a matter of debate. The situation was further complicated by the finding that tumour necrosis factor .alpha.(TNF.alpha.) is capable of stimulating the differentiation of the human myeloblastic cell line ML-1 (Takeda, K., Iwamoto, S., Sugimoto, H., Takuma, T., Kawatani, N., Noda, M., Masaki, A., Morise, H., Arimura, H. and Konno, K. (1986) Nature 323, 338-340).
In an attempt to resolve the discrepancies between the data of these groups, the present inventors have biochemically fractionated the medium conditioned by Krebs II ascites tumour cells used in a number of previous studies and shown it to contain not only authentic G-CSF and GM-CSF, active on normal progenitor cells as well as WEHI-3B D.sup.+ cells, but also two biochemically distinct, but functionally similar, factors capable of inducing the differentiation of M1 cells. These latter factors have been termed leukaemia inhibitory factor (LIF)-A and LIF-B because of their ability to suppress the proliferation of M1 leukaemic cells in vitro and it has been shown that they do not induce the differentiation of WEHI-3B D.sup.+ cells and do not stimulate the proliferation of normal granulocyte/macrophage progenitor cells.